Engine faults P0234 and P1351 on 1.6 HDi

Quite regularly (once a month or so) when I am driving quite hard on the motorway (say uphill at 70mph) my Xsara Picasso 1.6HDi Exclusive 2005 gives an engine fault and goes into limp mode. I can read the fault codes and there is always a single occurence of P0234 ('Turbocharger / Supercharger "A" Overboost Condition') and P1351 (don't know what that means). The car has done 110,000 miles.

Please can anyone tell me what P1351 means?

What could be the reason for these fault codes?

If the culprit is the EGR valve, should I clean it by some means, or have it replaced?